[Dazzanova]

Purpose

Part of #58647 (item 10) — system text blocks in the Anthropic
/v1/messages converter get joined with "".join(system_parts),
so multiple blocks run together with no separator, e.g.
...for Claude.You are.... Happens both with a multi-block
top-level system field and when inline system messages get
merged into the leading block for system-first chat templates.

Changed the separator to "\n\n" so blocks read as separate
paragraphs, the way Claude Code and other Anthropic clients
actually send them.

Used Claude to help identify and implement this fix and draft the
tests — reviewed and ran everything locally before submitting.

Test Plan

Added tests for the multi-block and merge_inline_system cases, and
fixed two existing tests that had the old no-separator output
hardcoded as the expected result.

Test Result

pytest tests/entrypoints/anthropic/test_anthropic_messages_conversion.py -v
— 83 passed, 0 failed.

Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

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